feat(cases): clone a Git repository as a new case (#236)

Adds an Add Case -> "Clone Repo" tab plus two endpoints, implementing
@DodgyBadger's proposal in #236: clone a public repository straight into
codeman-cases/<name> and register it as a normal local case.

POST /api/cases/clone is synchronous by design (request held open, bounded
by GIT_CLONE_TIMEOUT_MS): no job store, no polling, no cancellation
surface. Success broadcasts the usual case:created event, so the case
still appears when a proxy idle-timeout kills the request mid-clone.

POST /api/cases/clone-preflight runs `git ls-remote --symref` so the UI can
say, while the user is still typing, whether the URL is cloneable without
credentials, what its default branch is, and which branches/tags exist.

Core lives in src/git-clone.ts, split into a pure half (URL parse, argv/env,
ls-remote parse, stderr classification) and a thin IO half, so every
security decision is unit-testable without spawning anything:

- `<name>::<payload>` transports are refused as a family, not by name:
  ext:: is the famous one, but any of them dispatches to git-remote-<name>
  and turns a clone into arbitrary command execution.
- A leading `-` is refused AND every spawn puts `--` before the operands.
  Either alone is one edit away from being a hole.
- argv arrays, never a shell. URLs carrying user:password@ are refused.
- gitNonInteractiveEnv() closes all four ways git can block on a prompt
  with no terminal attached (terminal prompt, askpass/GUI, ssh, GCM).
  HOME/PATH stay inherited, so a user's own credential helper or ssh agent
  keeps working; Codeman itself collects and stores nothing.
- The timeout signals the process GROUP, since clone fans out into
  git-remote-https/index-pack children that outlive a signal to the parent.
- Bounded output (redacted stderr tail, capped ls-remote stdout, 500 refs
  each) and a global 2-op pool, so N large clones cannot exhaust the host.

Repository contents beat scaffolding: an existing CLAUDE.md is kept, hooks
are merged into whatever .claude/settings.local.json the repo shipped, and
a repo that ships its own Claude settings is reported back as a warning
(those hooks run locally as soon as a session starts there). A failed clone
removes only the directory the attempt created, and refuses a pre-existing
destination outright, so it can never squat on a case name.

Not admin-gated in multi-user mode, unlike /api/cases/link: it writes only
inside the caller's own case space. Local-path/file:// sources are the
exception and stay admin-only there.

UI: live verdict under the URL field, case name filled from the parsed repo
until the user types their own, branch/tag as a datalist of the remote's
real refs, optional shallow clone, and a Brain picker (installed CLIs only)
that points the Run button at the chosen agent. Starting a session stays
opt-in. The tab hides itself when the server reports no git.

Tests: the pure half exhaustively (every refusal has a case), plus real git
against a real local bare repo for clone/ref/timeout/cleanup, and a
route-level suite with unmocked fs that clones through the endpoint.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

test/render-index-html.test.ts

@@ -18,6 +18,7 @@ import { isCodexAvailable } from '../src/utils/codex-cli-resolver.js';
 import { isGeminiAvailable } from '../src/utils/gemini-cli-resolver.js';
 import { isAntigravityAvailable } from '../src/utils/antigravity-cli-resolver.js';
 import { isCloudflaredAvailable } from '../src/utils/cloudflared-resolver.js';
+import { isGitAvailable } from '../src/git-clone.js';
 
 // renderIndexHtml probes the real PATH for every CLI, which would make the
 // assertions below depend on whatever happens to be installed on the machine
@@ -46,6 +47,10 @@ vi.mock('../src/utils/cloudflared-resolver.js', () => ({
   isCloudflaredAvailable: vi.fn(() => false),
   resolveCloudflaredPath: vi.fn(() => null),
 }));
+// git gates the Add Case -> Clone Repo tab (#236), so it rides in the same object.
+vi.mock('../src/git-clone.js', () => ({
+  isGitAvailable: vi.fn(() => false),
+}));
 
 const TEMPLATE = [
   '<head>',
@@ -127,6 +132,7 @@ describe('WebServer.renderIndexHtml', () => {
     vi.mocked(isGeminiAvailable).mockReturnValue(false);
     vi.mocked(isAntigravityAvailable).mockReturnValue(false);
     vi.mocked(isCloudflaredAvailable).mockReturnValue(true);
+    vi.mocked(isGitAvailable).mockReturnValue(true);
     const { server } = makeServer({});
     const html = await render(server);
     const flags = JSON.parse(html.match(/window\.__codemanCliAvailable=(\{.*?\});/)![1]);
@@ -139,6 +145,7 @@ describe('WebServer.renderIndexHtml', () => {
       gemini: false,
       antigravity: false,
       cloudflared: true,
+      git: true,
     });
   });
 
@@ -152,6 +159,7 @@ describe('WebServer.renderIndexHtml', () => {
       isGeminiAvailable,
       isAntigravityAvailable,
       isCloudflaredAvailable,
+      isGitAvailable,
     ]) {
       vi.mocked(probe).mockReturnValue(false);
     }